This invention relates to apparatus producing a machine-direction intermittent heat seal for sealing a hem in a moving web of film and more particularly to the manufacture of draw tape plastic bags.
Bags made of thin polyethylene materials have been used in various sizes. Small bags are used in the packaging of samples and the like. Larger bags are used as shopping bags; and even larger bags are used for containing trash.
A particularly advantageous closure for such a bag includes a draw tape constructed from the same polyethylene material. U.S. Pat. No. 3,029,853--Piazzi and British Pat. No. 1,125,363--Jortikka are examples of draw tape bags. Such closures have been successfully employed on these bags.
Draw tape closures for large trash bags, and the manufacture of these draw tape trash bags, are described in the related applications identified below.
Forming the hem, into which the tape is inserted is shown for example, in U.S. Pat. Nos. 2,897,729--Ashton et al 3,058,402--Kugler, 3,058,403--Kugler, 4,558,463--Boyd, and in copending patent applications of applicant's assignee including "METHOD AND APPARATUS FOR FORMING HEMS IN SUPERPOSED PLIABLE PANELS," Boyd et al, Ser. No. 652,255, filed Sept. 20, 1984, now U.S. Pat. No. 4,617,008 and in "HEM FOLDER WITH INTEGRAL TAPE INSERTER FOR MAKING DRAW TAPE BAGS," Herrington, Ser. No. 871,238 filed 6/6/86.
Draw tape hems have been previously made by various methods. One method is to use a Teflon-coated hot bar which opens and closes against the film in the area of the bag machine where the film starts and stops. The bar closes once during each cycle while the film is stopped. Either continuous or intermittent seals may be formed by this method. A second method is to blow a series of hot air jets against the film as it travels by. Bag closures for use in the field use a pair of heated belts to produce a machine-direction seal. In the hot bar method, the hot bar is located far downstream from where the tape is inserted into the hem and also where the hem is folded so there has been an opportunity for the tape and the hem to wander by the time it reaches the sealer. This requires making the hem wider to allow for this wandering movement. This is costly in wasted material, produce a weak tape seal at the side edges of the bag, and also makes a sloppy bag. The hot air sealer can be used on continuously moving web, but it is very sensitive to wrinkles. The heating of the film by the air jets also causes some wrinkling which cannot easily be prevented since the film is not mechanically constrained. As a result the hem seals made by these techniques are not reliable, particularly where thin films, in the order one mil, are used.
It is desirable from an economic standpoint that the heat seals be produced while the web of film is moving. Apparatus for heat sealing hems in a web of film moving at a relatively high speed, such as 250-300 feet per minute, are disclosed in the related patent applications identified below. In those applications the heat seal was made with a hot wheel sealer and the seal was continuous throughout the length of the hem. It has been found that a moving web of film is subjected to some transverse as well as machine direction tensions. A transverse tension is sometimes concentrated in local areas, for example adjacent to punched holes through which the draw tapes can be grasped. When the film is heated to make a machine direction seal, it is weakened along a continuous line. Transverse tensions stretch the weakened seal, making it thin and weak, particularly in areas of high transverse tension, such as by the hole. By providing an intermittent or discontinuous heat seal the small interruptions in the heat seal allow the film to retain its integrity so the seal will not become stretched and weak. The gaps in the intermittent seal provide a strong tie across the hot seal, preventing it from stretching. It is an object of the present invention to provide apparatus for producing a machine-direction intermittent heat seal in a moving web of plastic film where the interruptions in the heat seal are relatively small, for example about 1/4" long, at intervals of about 3/4", resulting in a discontinuous seal having seal lengths about 1/2" long with 1/4" gaps.
It is a further object of the present invention to seal hems with intermittent heat seals in the opposing panels of a moving web of film in the manufacture of draw tape bags.